A laser is a device that produces optical radiation through the process of amplified stimulated emission in an excited medium. A laser resonator is a laser device that uses positive optical feedback in conjunction with an excited lasing medium to build up large optical field intensities either from radiation noise or from an injected lower power optical field. The allowable frequencies of optical radiation within a laser resonator are termed "longitudinal modes" and are given by the equation .nu..sub.m =mc/2nl, where .nu..sub.m is the frequency of optical radiation for the m.sup.th order longitudinal mode, c is the vacuum speed of light, n is the index of refraction of the medium (assuming it to be uniform within the resonator), and I is the physical length of the resonator optical cavity, i.e. the separation between the resonator mirrors. The spacing between adjacent longitudinal modes is therefore .DELTA..nu.=.nu..sub.m -.nu..sub.m-1 =c/2nl. Replacing the quantity, nl, by a single parameter, L, which is termed the "optical length" of the resonator, gives .DELTA..nu.=c/2L. The optical length is a useful parameter even when more than one optical medium is present within the laser resonator.
Very short laser resonator cavities, i.e. small L, have been used to produce large spacings between longitudinal modes. "Microlasers", also called "microchip lasers," have been developed which use a very short laser crystal that is closely coupled to a laser diode. The laser diode excites the ions within the lasing medium through optical pumping on an absorption line. The transverse mode structure is determined by the size of the region within the laser crystal that is pumped, also called the "pumped volume," relative to the region in which a particular mode is confined, also called the "mode volume."
Every laser medium has a "lineshape function" for each allowable energy transition which represents the probability that a photon of a given frequency will be emitted spontaneously for that particular higher-to-lower energy transition. The width of the lineshape function is a characteristic of the lasing medium and is determined by one or more mechanisms which may act on each species (atom, ion, or molecule) in exactly the same way (homogeneous broadening) or treat each in a different way (inhomogeneous broadening). The height of the lineshape function is a measure of the amplification or optical gain that the laser provides as a function of the optical frequency. The highest gain occurs at the peak of the lineshape function.
A laser resonator surrounding a homogeneously broadened laser medium will lase simultaneously in all longitudinal modes of the resonator above the lasing threshold, which is the point at which the round trip optical gain in the laser medium just equals the round trip optical loss within the resonator due to absorption and mirror reflectivity. If the intermode spacing for the resonator, .DELTA..nu., is smaller than the width of the lineshape function, it is possible for the laser to lase on multiple longitudinal modes. For continuous (or CW) lasing, however, the gain is clamped to its threshold value and only those longitudinal modes right at the peak of the lineshape function will experience a net gain and lase. This is not a stable situation, because the gain for the various longitudinal modes vary in a random manner due to, for example, resonator length changes caused by room vibration. This variation in mode gain results in random mode hopping, as low amplitude modes can receive sufficient gain to grow and become dominant at the expense of the previously dominant modes. Because of this intense mode competition during continuous lasing, few modes ever lase at the same time.
A Q-switched laser is one in which the loss within the cavity is maintained in a high state (low Q) during the time the medium is being excited, allowing the gain to build to a high value, and then abruptly switched to a low state, at which time lasing occurs and a high energy pulse is extracted.
In a gain switched laser, the optical gain is changed from a below-threshold level to an above-threshold level and back down in order to generate the laser pulse. If the pumping rate is sufficiently high, the gain can be forced high enough above threshold in a short enough period of time that multiple longitudinal modes will build from noise and lase before gain clamping has had time to cause mode discrimination. The output power in each mode will vary somewhat for a variety of reasons. First, the differential gain between modes will cause the output power to follow the lineshape function. Modes further from line center will therefore have lower output power. Second, because oscillation starts from noise with each longitudinal mode starting from independent stray photons, the output power will be higher in modes that start early. The time it takes to seed a mode, relative to the pulse buildup time, therefore, helps determine the magnitude of the variation in power from mode-to-mode. Third, over time the lasing process will clamp the gain at the threshold value due to depletion of the population inversion within the medium and a single dominant mode will emerge as described above in the CW lasing case.
Both Q-switched and gain switched laser resonators can be forced to operate on a single longitudinal mode by injection seeding the resonator with an external laser beam that is tuned to the same frequency as the desired longitudinal mode. Alternatively, the frequency of the external seed laser beam can be fixed and the length of the original resonator adjusted to control the frequency of the desired mode so that it coincides with the fixed frequency of the seed laser. In this case the original resonator is termed a "slave resonator" because its desired resonant frequency is slaved to the seed laser. In both cases, lasing will occur only in the desired longitudinal mode because the buildup time from the seed beam is so much faster than any other unseeded mode that must build up from random noise photons.
While some applications of lasers prefer emission on a single longitudinal mode, there exist other applications for which simultaneous emission on several longitudinal modes is desirable. Active tracking and wavefront sensing systems using speckle averaging at a remote target to minimize amplitude jitter in the return signal are two examples of applications for which multiple emission modes are desirable. For many of these applications, it is desirable to have high optical quality beams, a broad range in frequency across the several modes (i.e. short temporal coherence length), high power output, and stable operation of the several modes simultaneously.
Master Oscillator Power Amplifier (MOPA) laser topology has been used to provide high optical quality and high output power laser beams. Nonlinear optical phase conjugation is often used with the MOPA topology to correct thermally induced optical distortions in the power amplifier elements. The theory of operation of a phase conjugate MOPA is described in Koechner, W., Solid-State Laser Engineering, Second Edition, pp. 535-539, Springer-Verlag, Berlin, 1988.
The phase conjugate mirror components typically used in short pulse MOPA topologies are based on stimulated Brillouin scattering (SBS). The SBS process is well known in the art and is also described in Koechner. Because the acoustic waves in the Brillouin medium are receding, the optical frequency of the output beam will be shifted to a longer frequency than the oscillator beam due to the Doppler effect. The Brillouin medium can be a solid, liquid, or gas, and the magnitude of the Stokes shift in optical frequency is dependent on the speed of sound in the medium.
For highly distorting power amplifier media, such as highly pumped rod-shaped laser crystals that are actively cooled along the cylindrical surface of the rod, it is advantageous to confine the laser beam while propagating within the Brillouin medium using an optical lightguide. The incident beam is focused into a Brillouin medium such that the electrical field strength of the optical beam within the medium is sufficient to cause motion of the atoms or molecules through electrostriction. This sets up a series of receding acoustic waves in the medium that scatter the incident beam in the backward direction. The phase fronts of the acoustic waves have exactly the same shape as the phase front of the incident optical beam, therefore the backscattered beam will also have the same phase front but will be traveling in the opposite direction. This process of phase front reversal is known as phase conjugation and has the property that any optical ray entering the phase conjugate mirror will be retroreflected, i.e., will retrace its propagation path back to the source. This approach has been shown to provide good phase conjugate mirror reflectivity and fidelity with incident beams up to 75 times the diffraction limit. The lightguide approach is also advantageous when using the method of Basov described in Basov, N. G., Efimkov, V. F., Zubarev, I. G., Kotov, A. V., Mikhailov, S. I., and Smirnov, M. G., "Inversion of Wavefront in SMBS of a Depolarized Pump" (JTEP Letters, Vol. 28, No. 4, pp. 197-201, August 1978) to correct for depolarization of the laser beam in the power amplifier caused by thermally induced stress birefringence.
Lightguide phase conjugate mirror arrangements operate most efficiently and effectively when the temporal coherence length of the incident beam is longer than the round trip propagation time in the lightguide. Longitudinal modes that are separated in frequency by more than c/I.sub.c, where c is the speed of light and I.sub.c is the temporal coherence length, are treated as independent beams within the phase conjugate mirror and each must well exceed the conjugation threshold to be effectively conjugated.